There has long been a disparity in the corrosion resistance of various portions of an assembled automotive vehicle. Areas accessible to modern coatings systems benefit from galvanized substrates, well developed zinc phosphate pretreatments, purpose-designed electrocoats and suitable topcoats that withstand long term weathering and corrosive environments.
In contrast to the superb performance of coatings on exposed areas which are readily accessible to cleaning, pretreating and coating, there are small areas of a vehicle which are problematic. In an automotive body shop, oily cold rolled and galvanized steels are formed, folded, and welded together from approximately 250 pieces of sheet steel to make a “body in white”, so named for its light colored appearance. In this assembled vehicle there are joints, seams and folded hems that are remarkably inaccessible to processing fluids such as alkaline cleaners, pretreating chemicals, and electrocoats, and hence vulnerable to corrosion. For welded seams, slight variations in part dimension and in heat distortion from spot welding result in gap volumes which are high in both aspect ratio and in variability.
Various processes are in place to provide generally adequate corrosion resistance to seams and hems on a vehicle body. High viscosity structural adhesives designed for peel strength and corrosion resistance provide a measure of protection. They are applied, typically as a bead, to oily metal in the body shop before the piece is folded, bent, or welded into an inaccessible enclosed volume. If the currently available structural adhesives could be applied consistently to perfectly fill the void volume formed by hems, seams, and the like, results would be fully satisfactory. The variation in gap volume in high speed production prevents such a perfect application of adhesive or sealant. There are often gaps, or “holidays”, that form where the adhesive does not reach and bare metal is exposed. Production economics preclude a process of applying an excess and wiping off material that squeezes out.
It would be desirable to provide a film-forming composition which can be applied to such substrate surfaces that are exposed to voids during piece shaping and joining, demonstrating enhanced corrosion resistance. It would be additionally desirable for such compositions to demonstrate low V.O.C., compatibility with (i. e., an ability to be applied by) existing robots in OEM body shops to minimize equipment retro-fitting, and an ability to withstand pretreatment chemicals.